The present invention is directed to drive mechanisms for web material. It is concerned particularly with such mechanisms in which a power source drives the web material in one direction but is disengaged when the web material is to be drawn in the other direction.
In devices such as electrostatic printer-plotters, a drive roll driven by a motor draws web material such as paper from a supply roll past a printing mechanism in a forward direction. Occasionally, it is desired to rewind the web material in a reverse direction, and it is beneficial for the drive-roll motor to be decoupled from the drive roll during this time so that the drag of the motor does not present a load to the rewinding mechanism.
Mechanisms for achieving this result have been used in the past. For instance, electrically activated solenoids have been used to couple the motor to the drive roller for forward driving and to decouple the motor for rewinding. For small printers, however, the power required to operate the solenoid is significant, so it is preferable to avoid the use of solenoids.
Other prior-art devices have avoided solenoids by using an arrangement in which a pivot arm carries one of the gears in the drive train. The pivot arm pivots between two positions. In one position, the gear that it carries engages the next gear in the train to drive the roll. In the other position, the carried gear is spaced from the above-mentioned next gear. It is thus decoupled from that gear in the other position, although it may engage a gear in another drive train. By frictionally engaging the pivot arm with a shaft or gear in the drive train, operation of the motor in one direction causes the pivot arm to pivot so as to cause engagement of the gear that it carries, while rotation of the motor in the opposite direction causes disengagement.
This arrangement enables the motor and drive roll to be decoupled by simply reversing the motor. However, although such an arrangement eliminates the need to provide power to a solenoid, the friction between the pivot arm and the drive train increases the load on the motor when the motor drives the drive roll.